Natural killer (NK) cells are naturally occurring lymphoid cells that are spontaneously cytotoxic in vitro to a variety of cultured tumor-cell lines, and recent evidence suggests that NK cells may play a major role in immunological defenses against developing cancers. Because NK cells have significant lytic potential, it would seem probable that their activities would be closely regulated in vivo. Insights into how NK cells are regulated in vitro may be crucial to understanding why NK cell activities may not be effective against certain types of tumors in vivo, thus leading to significant improvements in future approaches to clinical immunotherapy. We have found that human NK cells can be regulated in vitro by: (1)\interactions with other cellular elements derived from human peripheral blood (especially autologous granulocytes and erythrocytes); (2)\soluble factors found in the serum (especially transferrin and transferrin-related proteins); and (3)\self-defensive antigen expression on the tumor targets themselves (especially Class II antigens). These aspects are being studied in a 4 hr, 51Cr-release assay that uses theK562 erythroleukemia cell line, as well as MOLT-4, CEM, Daudi, Raji, and LB as tumor-target cells. Our data have also shown that: (1)\cytotoxic T cells are also inhibited by autologous granulocytes; (2)\concentrated cytosol of autologous granulocytes inhibits NK cells as well as intact granulocytes; (3)\very low numbers of exogenous granulocytes or RBCs can very strongly enhance the in vitro cytolytic activity of low numbers of NK cells (this phenomenon seems to be a membrane-related event); (4)\NK lytic activity very clearly does not require or involve an oxidative burst; (5)\the presence of lithium salts in a culture of NK cells seems to prolong their cytolytic capacity compared to controls; and (6)\products secreted by Bordetella pertussis and B. parapertussis strongly inhibit NK function in vitro. (SR)